Technical Field
The present disclosure generally relates to the field of microelectronic sensors and, in particular, gas sensors integrated with circuitry on a semiconductor substrate.
Description of the Related Art
Miniature solid state gas sensors integrated with microelectronics allow construction of an “electronic nose” that can selectively detect the presence of, for example, toxic substances such as carbon monoxide (CO), or vapor associated with controlled substances such as ethanol.
It is well known that thin films made of certain metal materials, for example, semiconductor metal oxides (SMOs), experience a change in resistivity when they are exposed to certain gases at certain temperatures, as described in U.S. Pat. No. 4,485,667. Typically, SMO sensors operate at temperatures between 200 and 500 C. One such example of an SMO material is tin oxide (SnO2) which, when heated to 380 C-400 C and then exposed to methane gas, experiences a chemoresistive reaction (1) that produces free electrons, thereby altering the resistivity of the tin oxide film:SnO2+CH4→CO2+H2O+e−  (1)SMO sensors based on SnO2 or nickel oxide (NiO) are currently manufactured and sold by companies such as Hanwei Electronics Co., Ltd. of Zhengzhou in the Henan province of China, and Figaro Engineering, Inc. of Glenview, Ill.
By integrating a tin oxide thin film with an integrated circuit and a heating element, it is possible to construct an electronic gas detector suitable for use in a home or industrial environment, for example, or a portable breathalizer for use by law enforcement officers. Integration of chemical sensors with microelectronics on a common semiconductor die is described in U.S. Patent Application Publications 2012/0168882 and 2012/0171713, to Cherian and LeNeel, a co-inventor of this patent application, which publications are hereby incorporated by reference in their entirety.
One problem that arises in designing SMO thin film gas sensors is that the associated heaters tend to consume large amounts of electrical power to heat the SMO thin films to operating temperatures in the range of about 100 C-500 C, so the heaters tend to quickly drain the battery of the portable sensor device.
Another challenge is that the SMO sensors may have difficulty distinguishing between two gases. For example, carbon monoxide (CO) sensors also tend to be sensitive to hydrogen gas (H2). Thus, it is desirable to operate the sensor only within a small temperature range around the temperature at which the sensor has peak sensitivity to the particular gas of interest. Furthermore, instead of heating one sensor to different temperatures to detect different gases, it would be advantageous to dedicate specific sensors to specific gases. Alternatively, it would be advantageous to assemble multiple data points from a plurality of sensors that are all tuned to a particular gas, to increase specificity and to obtain more precise measurements.
Another challenge lies in providing thermal insulation for the gas sensors so that neighboring devices are not heat-damaged by such extreme temperatures. In some environments, heating the SMO sensor to temperatures in the range of 100 C-400 C can pose a safety risk—for example, if there exists a sufficient concentration of an ambient gas for which the combustion temperature is within the range of the operating temperature of the sensor. Some existing SMO sensor products include insulating layers between the heater and the substrate. U.S. Patent Application Publication 2009/0243003, entitled “Manufacturing Method of a Gas Sensor Integrated on a Semiconductor Substrate,” assigned to the same assignee as this patent application, addresses thermal insulation by forming an insulating cavity buried in the semiconductor substrate, the cavity being filled with air. However, although air is an effective thermal insulator, a substrate having an air cavity is generally structurally unstable, and prone to collapse. Thus, the depth of such an air cavity may be structurally limited.